Finding May Solve Riddle of Fatigue in Muscles - New York Times
http://www.nytimes.com/2008/02/12/health/research/12musc.html?_r=2&oref=slogin&oref=slogin
"One of the great unanswered questions in physiology is why muscles get tired. The experience is universal, common to creatures that have muscles, but the answer has been elusive until now."